Wednesday, August 4, 2010

Using NASA satellite data, scientists have produced a first-of-its kind map that details the height of the world’s forests. Although there are other local- and regional-scale forest canopy maps, the new map is the first that spans the entire globe based on one uniform method.

The work -- based on data collected by NASA's ICESat, Terra, and Aqua satellites -- should help scientists build an inventory of how much carbon the world’s forests store and how fast that carbon cycles through ecosystems and back into the atmosphere. Michael Lefsky of the Colorado State University described his results in the journal Geophysical Research Letters.

The new map shows the world’s tallest forests clustered in the Pacific Northwest of North America and portions of Southeast Asia, while shorter forests are found in broad swaths across northern Canada and Eurasia. The map depicts average height over 5 square kilometers (1.9 square miles) regions), not the maximum heights that any one tree or small patch of trees might attain.

Temperate conifer forests -- which are extremely moist and contain massive trees such as Douglas fir, western hemlock, redwoods, and sequoias--have the tallest canopies, soaring easily above 40 meters (131 feet). In contrast, boreal forests dominated by spruce, fir, pine, and larch had canopies typically less than 20 meters (66 feet). Relatively undisturbed areas in tropical rain forests were about 25 meters (82 feet), roughly the same height as the oak, beeches, and birches of temperate broadleaf forests common in Europe and much of the United States.

Scientific interest in the new map goes far beyond curiosities about tree height. The map has implications for an ongoing effort to estimate the amount of carbon tied up in Earth’s forests and for explaining what sops up 2 billion tons of “missing” carbon each year.

Humans release about 7 billion tons of carbon annually, mostly in the form of carbon dioxide. Of that, 3 billion tons end up in the atmosphere and 2 billion tons in the ocean. It’s unclear where the last two billion tons of carbon go, though scientists suspect forests capture and store much of it as biomass through photosynthesis.

There are hints that young forests absorb more carbon than older ones, as do wetter ones, and that large amounts of carbon end up in certain types of soil. But ecologists have only begun to pin down the details as they try to figure out whether the planet can continue to soak up so much of our annual carbon emissions and whether it will continue to do so as climate changes.

“What we really want is a map of above-ground biomass, and the height map helps get us there,” said Richard Houghton, an expert in terrestrial ecosystem science and the deputy director of the Woods Hole Research Center.

One of Lefsky’s colleagues, Sassan Saatchi of NASA’s Jet Propulsion Laboratory, has already started combining the height data with forest inventories to create biomass maps for tropical forests. Complete global inventories of biomass, when they exist, can improve climate models and guide policymakers on how to minimize the human impact on climate with carbon offsets.

More immediately, said University of Maryland remote sensing expert Ralph Dubayah, tree canopy heights can be plugged into models that predict the spread and behavior of fires, as well as ecological models that help biologists understand the suitability of species to specific forests.

Lefsky used data from a laser technology called LIDAR that’s capable of capturing vertical slices of surface features. It measures forest canopy height by shooting pulses of light at the surface and observing how much longer it takes for light to bounce back from the ground surface than from the top of the canopy. Since LIDAR can penetrate the top layer of forest canopy, it provides a fully-textured snapshot of the vertical structure of a forest -- something that no other scientific instrument can offer.

“LIDAR is unparalleled for this type of measurement,” Lefsky said, noting it would have taken weeks or more to collect the same amount of data in the field by counting and measuring tree trunks that LIDAR can capture in seconds.

He based his map on data from more than 250 million laser pulses collected during a seven year period. That may sound like an enormous amount of data, but each pulse returns information about just a tiny portion of the surface. Overall, the LIDAR offered direct measurements of 2.4 percent of the Earth’s forested surfaces.

To create his global map forest height map, Lefsky combined the LIDAR data with information from the Moderate Resolution Imaging Spectroradiometer (MODIS), a satellite instrument aboard both the Terra and Aqua satellites that senses a much broader swath of Earth’s surface, even though it doesn’t provide the vertical profile.

“This is a really just a first draft, and it will certainly be refined in the future,” said Lefsky.

Fusing the two sets of data proved difficult, and Lefsky spent years honing quantitative techniques to make the combination possible. Part of the difficulty was that the LIDAR data Lefsky used came from an instrument aboard ICESat, a mission optimized to study the topography of ice sheets, not vegetation.

The next generation LIDAR measurements of forests and biomass, which will improve the resolution of the map considerably, could come from NASA's Deformation, Ecosystem Structure and Dynamics of Ice (DESDynI) satellite, proposed for the latter part of this decade.

“We’ve never been able to look at a map and say here’s how tall the canopy is before,” said Dubayah, one of the DESDynI project scientists. “This map is a big step forward, and it really helps set the stage for DESDynI and shows what’s possible.”

(Photo: NASA Earth Observatory/Image by Jesse Allen and Robert Simmon/Based on data from Michael Lefsky)

UA scientists have achieved a breakthrough in the fight against malaria: a mosquito that can no longer give the disease to humans.

For years, researchers worldwide have attempted to create genetically altered mosquitoes that cannot infect humans with malaria. Those efforts fell short because the mosquitoes still were capable of transmitting the disease-causing pathogen, only in lower numbers.

Now for the first time, University of Arizona entomologists have succeeded in genetically altering mosquitoes in a way that renders them completely immune to the parasite, a single-celled organism called Plasmodium. Someday researchers hope to replace wild mosquitoes with lab-bred populations unable to act as vectors, i.e. transmit the malaria-causing parasite.

"If you want to effectively stop the spreading of the malaria parasite, you need mosquitoes that are no less than 100 percent resistant to it. If a single parasite slips through and infects a human, the whole approach will be doomed to fail," said Michael Riehle, who led the research effort, the results of which were published July 15 in the journal Public Library of Science Pathogens.

Riehle is a professor of entomology in UA's College of Agriculture and Life Sciences and is a member of the BIO5 Institute.

Riehle's team used molecular biology techniques to design a piece of genetic information capable of inserting itself into a mosquito's genome. This construct was then injected into the eggs of the mosquitoes. The emerging generation carries the altered genetic information and passes it on to future generations.

For their experiments, the scientists used Anopheles stephensi, a mosquito species that is an important malaria vector throughout the Indian subcontinent.

The researchers targeted one of the many biochemical pathways inside the mosquito's cells. Specifically, they engineered a piece of genetic code acting as a molecular switch in the complex control of metabolic functions inside the cell. The genetic construct acts like a switch that is always set to "on," leading to the permanent activity of a signaling enzyme called Akt. Akt functions as a messenger molecule in several metabolic functions, including larval development, immune response and lifespan.

When Riehle and his co-workers studied the genetically modified mosquitoes after feeding them malaria-infested blood, they noticed that the Plasmodium parasites did not infect a single study animal.

"We were surprised how well this works," said Riehle. "We were just hoping to see some effect on the mosquitoes' growth rate, lifespan or their susceptibility to the parasite, but it was great to see that our construct blocked the infection process completely."

Of the estimated 250 million people who contract malaria each year, 1 million – mostly children – do not survive. Ninety percent of the number of fatalities, which Riehle suspects to be underreported, occur in sub-Saharan Africa.

Each new malaria case starts with a bite from a vector – a mosquito belonging to the genus Anopheles. About 25 species of Anopheles are significant vectors of the disease.

Only the female Anopheles mosquitoes feed on blood, which they need to produce eggs. When they bite an infected human or animal, they ingest the malaria parasite.

Once the Plasmodium cells find themselves in the insect's midgut, they spring into action. They leave the insect's digestive tract by squeezing through the midgut lining. The vast majority of Plasmodium cells do not survive this journey and are eliminated by the mosquito's immune cells. A tiny fraction of parasite cells, usually not more than a handful, make it and attach themselves on the outside of the midgut wall where they develop into brooding cells called oocysts.

Within 10-12 days, thousands of new Plasmodium cells, so-called sporozoites, sprout inside the oocyst. After hatching from the oocyst, the sporozoites make their way into the insect's salivary glands where they lie in wait until the mosquito finds a victim for a blood meal. When the mosquito bites, some sporozoites are flushed into the victim's bloodstream.

"The average mosquito transmits about 40 sporozoites when it bites," said Riehle, "but it takes only one to infect a human and make a new malaria victim."

Several species of Plasmodium exist in different parts of the world, all of which are microscopically small single-celled organisms that live in their host's red blood cells. Each time the parasites undergo a round of multiplication, their host cells burst and release the progeny into the bloodstream, causing the painful bouts of fever that malaria is known and feared for.

Malaria killed more soldiers in the Civil War than the fighting, according to Riehle. In fact, malaria was prevalent in most parts of the U.S. until the late 1940s and early 1950, when DDT spraying campaigns wiped the vectors off the map. Today, a new case of malaria occurs in the U.S. only on rare occasions.

The severity of the disease depends very largely on the species of the Plasmodium parasite the patient happens to contract.

"Only two species of Plasmodium cause the dreaded relapses of the disease," said Riehle. "One of them, Plasmodium vivax, can lie dormant in the liver for 10 to 15 years, but now drugs have become available that target the parasites in the liver as well as those in the blood cells."

That said, there are no effective or approved malaria vaccines. A few vaccine candidates have gone to clinical trials but they were shown to either be ineffective or provide only short-term protection. If an effective vaccine were to be developed, distribution would be a major problem, Riehle said.

Researchers and health officials put higher hopes into eradication programs, which aim at the disease-transmitting mosquitoes rather than the pathogens that cause it.

"The question is 'What can we do to turn a good vector into a bad vector?'" Riehle said.

"The eradication scenario requires three things: A gene that disrupts the development of the parasite inside the mosquito, a genetic technique to bring that gene into the mosquito genome and a mechanism that gives the modified mosquito an edge over the natural populations so they can displace them over time."

"The third requirement is going to be the most difficult of the three to realize," he added, which is why his team decided to tackle the other two first.

"It was known that the Akt enzyme is involved in the mosquito's growth rate and immune response, among other things," Riehle said. "So we went ahead with this genetic construct to see if we can ramp up Akt function and help the insects' immune system fight off the malaria parasite."

The second rationale behind this approach was to use Akt signaling to stunt the mosquitoes' growth and cut down on its lifespan.

"In the wild, a mosquito lives for an average of two weeks," Riehle explained. "Only the oldest mosquitoes are able to transmit the parasite. If we can reduce the lifespan of the mosquitoes, we can reduce the number of infections."

His research team discovered that mosquitoes carrying two copies of the altered gene had lost their ability to act as malaria vectors altogether.

"In that group of mosquitoes, not a single Plasmodium oocyst managed to form."

At this point, the modified mosquitoes exist in a highly secured lab environment with no chance of escape. Once researchers find a way to replace wild mosquito populations with lab-bred ones, breakthroughs like the one achieved by Riehle's group could pave the way toward a world in which malaria is all but history.

As physician-guided robots routinely operate on patients at most major hospitals, the next generation robot could eliminate a surprising element from that scenario -- the doctor.

Feasibility studies conducted by Duke University bioengineers have demonstrated that a robot -- without any human assistance -- can locate a man-made, or phantom, lesion in simulated human organs, guide a device to the lesion and take multiple samples during a single session. The researchers believe that as the technology is further developed, autonomous robots could some day perform many more simple surgical tasks.

“Earlier this year we demonstrated that a robot directed by artificial intelligence can on its own locate simulated calcifications and cysts in simulated breast tissue with high repeatability and accuracy,” said Kaicheng Liang, a former student in the laboratory of Stephen Smith, director of the Duke University Ultrasound Transducer Group at the Pratt School of Engineering and senior member of the research team. “Now we have shown that the robot can sample up to eight different spots in simulated human prostate tissue."

The results of the Duke research appear in the current issue of the journal Ultrasonic Imaging. An earlier study reported in the January issue of the journal Ultrasound in Medicine and Biology described the Duke team’s results on simulated breast tissue. In both experiments, whole turkey breasts were used. Raw turkey breasts are commonly used in medical research because the tissue closely resembles that of humans in texture and density, and appear similar when scanned by ultrasound.

The Duke team combined a “souped-up” version of an existing robot arm with an ultrasound system of its own design. The ultrasound serves as the robot’s “eyes” by collecting data from its scan and locating its target. The robot is “controlled” not by a physician, but by an artificial intelligence program that takes the real-time 3-D information, processes it and gives the robot specific commands to perform. The robot arm has a mechanical “hand” that can manipulate the same biopsy plunger device that physicians use to reach a lesion and take samples.

In the latest series of experiments, the robot guided the plunger to eight different locations on the simulated prostate tissue in 93 percent of its attempts. This is important because multiple samples can also determine the extent of any lesion, Smith said.

Smith believes that routine medical procedures, such as biopsies in other tissues in the body, will be performed in the future with minimal human guidance, and at greater convenience and less cost to patients.

An important challenge to be overcome is the speed of data acquisition and processing, though the researchers are confident that faster processors and better algorithms will address that issue. To be clinically useful, all of the robot’s actions would need to be in real time, the researchers said.

“One of the beauties of this system is that all of the hardware components are already on the market,” Smith said. “We believe that this is the first step in showing that with some modifications, systems like this can be built without having to develop a new technology from scratch.”

Advances in ultrasound technology have made these latest experiments possible, the researchers said, by generating detailed, 3-D, moving images in real-time. The Duke team has a long track record of modifying traditional 2-D ultrasound -- like that used to image babies in utero -- into the more advanced 3-D scans. The Duke lab invented the technique in 1991.

“We’re now testing the robot on a human mannequin seated at the examining table whose breast is constrained in a stiff bra cup,” Smith said. “The breast is composed of turkey breast tissue with an embedded grape to simulate a lesion. Our next step is to move to an excised human breast.”